<?php 
	$conn = oci_connect("caridad","caridad","localhost/xe");  
    if (!$conn) {
        $e = oci_error();
        trigger_error(htmlentities($e['message'], ENT_QUOTES), E_USER_ERROR);
    }
?>

	<?php 
		 $cat_colonias = oci_parse($conn,"SELECT ID, COLONIA, CODIGO_POSTAL FROM CAT_COLONIA WHERE IDMUNICIPIO=".$_GET['id']." ORDER BY COLONIA ASC");
    	 oci_execute($cat_colonias);

		// while (($arreglo = oci_fetch_array($cat_colonias, OCI_BOTH))){
		// echo $arreglo['MUNICIPIO'];
		// }
	?>

Colonias:
<select name="colonias">	
	<?php while (($arreglo = oci_fetch_array($cat_colonias, OCI_BOTH))){ ?>
	<option value="<?php echo $arreglo['ID']?>"><?php echo $arreglo['COLONIA']?></option>
<?php }?>
</select>